1. Field of the Invention
The present invention relates to an acceleration sensor, and more specifically, to a capacitive acceleration sensor CAS) with a low production cost for meeting market requirements.
2. Description of the Prior Art
An acceleration sensor is widely applied in seismology, automobile safety air bag, robotics, and so on. Currently, an acceleration sensor in common use includes a piezoresistive acceleration sensor, a piezoresistive acceleration sensor, a capacitive acceleration sensor, and a semiconductor acceleration sensor.
Additionally, because sizes of the acceleration sensors are reduced gradually, a micromachining technology is developed to manufacture various microsensors and microactuators that are integrated with micro electronic circuits to form a microsystem, which is generally called a micro electro-mechanical system (MEMS). The MEMS has an extremely small size and can be manufactured by utilizing batch production for reducing a production cost. In addition, the MEMS and a signal processing circuit can be simultaneously formed on a silicon wafer for forming a monolithic device, which can reduce a distance between an acceleration sensor and the signal Processing circuit and that is quite important for the acceleration sensor. As the acceleration sensor outputs a signal, the signal is firstly amplified by the signal processing circuit for preventing the signal from being disturbed by an ambient electromagnetic field, and the signal can be analog-to-digital (AID) converted by the signal processing circuit and be transmitted to a central processing unit. Therefore, as the distance between the acceleration sensor and the signal processing circuit is reduced, signal reliability can be greatly improved, and interconnecting lines and loads of central control systems can be effectively decreased. As a result, the acceleration sensor that is manufactured by use of MEMS is developed rapidly due to its advantages of good detection sensitivity and a low production cost. Additionally, among the above-mentioned kinds of acceleration sensors, the capacitive acceleration sensor has advantages of high detection sensitivity and low sensitivity to an ambient environment so that the capacitive acceleration sensor has become more and more popular in a market.
Please refer to FIG. 1. FIG. 1 is a sectional view of a conventional capacitive acceleration sensor 10. As shown in FIG. 1, the capacitive acceleration sensor 10 mainly comprises a semiconductor substrate 12, such as a single-crystal silicon substrate or a silicon-on-insulator (SOI) substrate, an epitaxial-silicon beam structure 14 having a movable section where a movable electrode 16 is located, an epitaxial-silicon supporter 18 formed on the semiconductor substrate 12 for fixing the beam structure 14 and forming a distance between the beam structure 14 and the semiconductor substrate 12, and a doped region 20 positioned in the semiconductor substrate 12 and below the moveable electrode 16. The movable electrode 16 and the doped region 20 together constitute a plate capacitor 22, and the movable electrode 16 functions as an upper electrode of the plate capacitor 22 while the doped region 20 is used as a lower electrode or a stationary electrode of the plate capacitor 22. Additionally, the capacitive acceleration sensor 10 further comprises a control circuit 24, such as a complementary metal-oxide semiconductor (CMOS) control circuit, positioned in the supporter 18 or on the semiconductor substrate 12. The CMOS control circuit 24 is electrically connected to the plate capacitor 22 and is mainly used to receive, process, and transmit signals output from the plate capacitor 22.
When a vertical acceleration force is applied on the capacitive acceleration sensor 10, a flexural vibration occurs in the movable section of the beam structure 14, thereby altering a capacitance of the plate capacitor 22. Thereafter, the control circuit 24 receives a signal output from the plate capacitor 22 and performs a signal process, such as signal amplification or temperature compensation, on the signal. Then, the control circuit 24 converts the signal output from the plate capacitor 22 into a differential signal that is eventually output from the control circuit 24. Since the differential signal corresponds to the applied acceleration force, the capacitive acceleration sensor 10 can utilize the CMOS control circuit 24 to detect variations of an electrostatic capacitance of the plate capacitor 22 for obtaining the applied acceleration force. Moreover, the capacitance of the plate capacitor 22 is only relative to physical parameters, so that the capacitive acceleration sensor 10 can be formed with a material having a low thermal expansion coefficient for improving its detection sensitivity.
As described above, the semiconductor substrate 12, the beam structure 14, and the supporter 18 are composed of single-crystal silicon or epitaxial silicon, so that the conventional capacitive acceleration sensor 10 has good detection sensitivity. However, costs of silicon wafers and epitaxial silicon are so high that it costs a lot to form the conventional capacitive acceleration sensor 10. As a result, it is an important issue to manufacture a capacitive acceleration sensor with a low production cost and a high quality.